June 2010 / Volume 62 / Issue 6|
Getting reaming right
By Alan Richter
| Appropriate scenarios for applying a reamer when finishing holes.
Courtesy of Komet of America
When machining a hole exactly to size, reaming is often a machinist’s best choice. It typically cannot be used to straighten holes, and can only remove a limited amount of stock, but when done right, reaming can be a fast, highly accurate process.
Performing reaming correctly, however, requires good preparation. Important factors include leaving the exact amount of stock after drilling; knowing when to ream twice, use a multifunction reamer or use another tool entirely; and specifying the right reamer for the job. Regarding the latter, many different styles of reamers are available—including adjustable, chucking and spiral flute reamers—and in many cases a special is the best tool for the job.
When reaming, preparation of the hole is what’s most important, according to Ryan Bysterbusch, group leader of design engineering for toolmaker Komet of America Inc., Schaumburg, Ill. That’s because a reamer enlarges a drilled hole to size by removing a small amount of stock and does not correct a hole. “The reamer follows the hole,” he said, but noted that a reamer helps to slightly improve straightness if the tool has front-cutting capability.
Courtesy of Alvord-Polk
Josh Lynberg, president of tool supplier Monster Tool Co., Vista, Calif., concurred that a “fair-quality” hole is needed before reaming. For example, he noted that when reaming a 0.240 "-dia. hole with a 0.250 " reamer, the hole will not be straightened if it tapers by 0.005 " or more.
If the hole shape or location needs correction, Will Nestor, a Phenix City, Ala.-based application/project sales engineer for toolmaker Mapal Inc., Port Huron, Mich., recommends boring. “But reaming will typically give you a quicker cycle time than boring,” he said, adding that a reamer is more commonly applied to achieve the proper hole size than impart a fine surface finish.
The surface finish obtainable when reaming generally depends on the workpiece material. The range for cast iron is 50 to 80 rms and 30 to 60 rms for steels, and a PCD reamer can impart a finish as fine as 8 rms in aluminum, according to Bysterbusch.
Sometimes a hole is bored prior to reaming, but that’s not mandatory. “Any hole that meets the minimal required amount of stock for the reamed size needs no other preparations before reaming,” Lynberg said. “If the finished hole has an extremely tight tolerance and the machine tool being used to create the hole is not of sufficient accuracy or rigidity, a bored hole helps keep the reamer aligned with the hole axis, assisting the machine tool to keep the hole straight.”Material Issues
The amount of stock remaining for reaming depends on hole quality and diameter. A rule of thumb is 0.010 " to 0.015 " should remain after drilling for reaming, except for small diameters, such as 1⁄32 ", which should have 0.003 " to 0.006 " of material for reaming, Lynberg noted. “A poorly drilled hole may need a little more material in order for the reamer to ‘clean up’ the hole walls,” he said.
Nestor provided a general DOC range from 0.0039 " to 0.0098 " for Mapal’s guide-padded reamers and up to a 0.0394 " DOC for a brazed-PCD fixed reamer, depending on the workpiece material. “For an aluminum engine component application, we’ve been able to remove up to 1mm per side in the hole,” he said, adding that a larger DOC is possible for a reamer with end-cutting geometry incorporated into the cutting edge.
In addition to aluminum, the stock allowance for magnesium is usually greater than it is for steel, cast iron, titanium and nickel-base superalloys, according to Bysterbusch. If the chip is too thick when reaming the latter materials, too much heat enters the chip, making it ductile and difficult to break, which creates a chip-removal problem. “You’re better off removing as little as you can.”
Others provide more hole-size ranges. Kevin Morrison, chief tooling engineer for Alvord-Polk Inc., indicated that the Millersburg, Pa.-based manufacturer of multiple-flute solid reamers recommends 0.003 " to 0.006 " of stock for reaming holes up to 3⁄32 " in diameter, 0.008 " to 0.010 " for holes larger than 3⁄32 " to ¼ ", 0.012 " to 0.015 " for ¼ " to ½ ", 0.017 " to 0.020 " for ½ " to 1 ", 0.020 " to 0.025 " for 1 " to 2 " and 0.030 " to 0.035 " for holes larger than 2 ". “It’s a sliding scale,” he said. “It doesn’t even work out as a percentage.”
Morrison added that the range is 0.002 " to 0.004 " for a hand reamer. “That’s about all the human body is geared up to do,” he said. “We don’t have the mechanical advantage to take more material out.”
Although some don’t consider material workhardening to be a significant concern when reaming, Nestor noted that it can create challenges. It’s desirable to take a light DOC, for example, when reaming titanium. A light DOC keeps chips thin for ease of evacuation when machining, for example, a small-diameter hole. “Machinability plays a big role in how you ream,” he said.Sizing the Hole
When too much stock remains for reaming after drilling, end users have the option of reaming twice. Monster Tool’s Lynberg suggests first applying a smaller reamer followed by a reamer of the required size. “Drilling to open up a hole usually causes drill failure due to unequal and uneven stress along the drill’s cutting edge and is not recommended,” he said. Lynberg added that plunge milling is also an option if the proper size endmill is available.
In addition, a step reamer with a roughing and finishing diameter might do the trick. “It depends on the material,” Bysterbusch said. “Once you get into some of the tougher steels, we have to look more in detail at the application.”
Nestor noted that another option is applying a fine boring tool. A fine boring tool has at least three guide pads and is capable of a significantly greater DOC than a reamer. Bore location corrections can also be achieved using a fine boring tool with a cycle time comparable to a guide-padded reamer, he added.
Part volume also dictates the acceptable solution. “You have to remember the customer always wants to decrease cycle time,” said Donato Pigno, product specialist for Komet. “He’ll typically use a roughing tool and a finishing tool.”
Alvord-Polk’s Morrison suggested that a core drill, which is a cross between a reamer and a drill, can solve the problem when there’s considerably more material to remove than is recommended. A core drill is designed to remove a large amount of material from a hole, but it will not produce a hole by itself. “There are times when a core drill imparts a suitable-enough surface finish that end users use one to finish a hole,” he said.Tool Considerations
When specifying a reamer, size is the only feature an end user needs to be concerned with unless he’s reaming an unusually deep hole that needs extra tool length, which calls for a special, according to Lynberg. “A properly manufactured reamer will do its job regardless of the material being cut.”
Courtesy of Monster Tool
Although Bysterbusch noted that “full-blown specials” constitute about 40 percent of Komet’s reamer sales, the company’s fixed, monoblock standards are “semispecials.” That’s because standard blanks are ground to size to meet a specific customer’s requirements once the company receives an order. The standard range is from 6mm to 110mm but specials cover a wider spectrum. “We ream everything from 1.5mm to over 300mm,” he said.
Size tolerance plays a role in selecting the style of reamer, according to Mapal’s Nestor. While a fixed tool is appropriate for a larger tolerance range, “any time you’ve got a ±5μm or tighter tolerance on your diameter,” he said, “you want an adjustable reamer.”
Nestor added that chucking reamers can be applied in a drill press, toolroom lathe or even a Bridgeport mill, but guide- padded and other high-performance reamers require a machine with a mechanical feed to provide consistent accuracy. An adequate coolant supply is also required for high-performance reaming. “Typically, soluble oils and semisynthetics work better, but we’ve been successful with synthetic coolant for guide-padded tools,” he said.
Hole interruptions, such as a keyway or cross-hole, also dictate a reaming tool’s requirements. When such a feature is present, Alvord-Polk’s Morrison recommends a spiral-flute reamer so the helix bridges the gap as the reamer rotates and the tool is supported at all times. “If you have a straight flute, every time the tooth comes around it catches and pounds like nobody’s business,” he said.
Regardless of the reaming application, some maintain only an actual reamer will do the job properly. “It is dangerous to say you just want to use a drill with a reaming quality,” said Komet’s Pigno. “You will never get the same result.” CTE
About the Author: Alan Richter is editor of Cutting Tool Engineering, having joined the publication in 2000. Contact him at (847) 714-0175 or firstname.lastname@example.org.
Courtesy of Komet of America
Reaming a 'slick as a whistle' finish
Komet of America Inc.
Magna Powertrain USA Inc.
Monster Tool Co.
YG-1 Tool Co.
Dreaming of not reaming
the size and surface finish requirements a reamer provides with just one tool can boost throughput and reduce costs. Although toolmakers have long promoted various drills to eliminate reaming, YG-1 Tool Co., Vernon Hills, Ill., says its Dream Drill also performs center drilling and position boring.
The drill, which is coated with titanium aluminum nitride, can achieve a reamed hole tolerance of M7, according to YG-1 (see Table). Solid-carbide through-coolant drills are available from 0.039 " to 0.787 " and indexable-insert, through-coolant I Dream Drills are available from 0.4724 " to 1.250 ". Drills are also available without coolant holes.
YG-1 offers two designs for machining materials up to 50 HRC: standard for steel, cast iron and similar alloys, and Inox for stainless steel and softer, more ductile alloys. “Small chip curls are always the goal,” said Al Zaitoon, YG-1’s sales and marketing manager.
The toolmaker reports that the Dream Drill’s “S” form web point thinning reduces axial thrust loading and stabilizes the point to produce accurate holes, and the 140° point angle slightly thickens the chip cross section, which helps to break the chips.
Zaitoon noted a Dream Drill’s penetration rate is three to five times faster than a conventional drill, and a 20.00- to 30.00-ipm feed is not uncommon when drilling a 0.250 "-dia. hole in low-alloy steel.
“Generally, the higher the speed, the more ductile the material becomes, therefore reducing the thrust required to drill,” he said, “and the size of the coolant holes are designed to give maximum pressure and volume to reduce heat problems.”
According to Zaitoon, the Dream Drill is suitable for replacing about 50 to 70 percent of reaming applications when drilling from a solid workpiece. Those include applications where the surface finish requirement is specified “as reamed” and the drill can achieve size tolerance.
Table: Drill diameter tolerances
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